From the wikia:
"Matatabi, being made out of blue flames, has an affinity for the Fire Release. Unique to the cat, it can produce powerful blue flames for offensive purposes, or it can create extremely powerful normal-coloured gigantic fireballs that can destroy large buildings and carve out huge rock formations with ease."
I don't think it grants the same to its host.
Passively, I think it does in fact enhances the user's own fire release, if Yugito's mouse hairball fire technique is anything to go by, though it's a bit of a long shot.
http://naruto.wikia.com/wiki/Yugito_NiiIt can also allow the user to turn their nails into claws. Overall though, that's not really as great as manipulating sand and magnet release, but since Shukaku is stated to be able to control the host while they sleep and its fuinjutsu is likely only usable while in the transformed state of Shukaku, it isn't completely overwhelming.
http://naruto.wikia.com/wiki/ShukakuI was never a fan of two weeks, cause it made the smaller tails seem like child's play. I mean two weeks and you master the Ichibi? Seems way to quick for me. Especially since with the Ichibi, it meant full mastery, sand manipulation, sealing jutsu, and magnet release all in two weeks.
Idk, I don't know how you'd rectify that though.
Well, mastering the tailed beast's abilities should be separated from full control if we were to take that into consideration. Control should be relatively swift considering SL shinobi nowadays, but mastering the abilities, as you say, should take time, and should be proportional to the amount of skills that the beasts directly and indirectly grant to their hosts.
Controlling the beast means that you won't go nuts when you go up in the number of tails you're using, but that shouldn't mean that at the end of the two weeks period you have completely mastered magnet release.